Indication

Contraindication

Principle

Adrenal glucocorticoid secretion is controlled by adrenocorticotrophic hormone (ACTH) released by the anterior pituitary. This test evaluates the ability of the adrenal cortex to produce cortisol after stimulation by synthetic ACTH (tetracosactide: Synacthen ®). In subjects with enzyme deficiency in the steroid synthetic pathway, cortisol may, or may not, be adequately secreted. However, there is excessive secretion of the precursor steroids before the defective enzyme. The commonest form of CAH is due to deficiency of 21-hydroxylase and in these subjects increased secretion of 17 OH-progesterone can be detected.

Side effects

There are rare reports of hypersensitivity reactions to Synacthen particularly in children with history of allergic disorders.

Preparation

There are no dietary restrictions for this test. This test should be performed in the morning as diurnal variations in 17 OH-progesterone have not been established in CAH.

Hydrocortisone should be omitted on the morning of the test.

Prednisolone shouled be stopped 24 hours before the test and recommenced after (unless instructed to the contrary).

Requirements

2 plain tubes and 1 EDTA tube

250 microgram Synacthen (1 vial)

the dose for children is 36 microgram/kg body weight up to a maximum of 250 microgram

Procedure

Interpretation

A normal cortisol response would be indicated by a rise in the cortisol concentration of the 30 min sample to greater than 600 nmol/L.

There are marked variations in 17 OH-progesterone throughout the menstrual cycle and "normal" values cannot exclude non-classical CAH. Heterozygotes for 21 OH-deficiency should have post-ACTH values of 17 OH-progesterone > 35 nmol/L (guide value). There is an overlap with normals and this test is not diagnostic.

Homozygotes for non-classical CAH may have normal baseline values for 17 OH-progesterone but will have exaggerated responses to ACTH of > 60 nmol/L (guide value).